Semiconductor devices become finer in structure as the integration of their integrated circuits is higher. As a result, optical proximity effects greatly influence differences between designed photomask-patterns, and photoresist patterns transferred from the designed photomask-patterns to a photoresist film on a device substrate.
Concretely, an originally designed right-angle corner of photomask-patterns actually becomes round on a photoresist film, or a line of photoresist patterns eventually formed is deformedly shorter in length at its end, or wider or narrower in width than that of the designed photomask-patterns.
Such a deviation of photoresist patterns from photomask-patterns deteriorates characteristics of semiconductor devices and/or the breaking down or forming of bridges of photoresist patterns causes substantially decline in yield rate of the semiconductor devices.
Thus, in order to obtain desired photoresist patterns, photomask-patterns are required to correct deviations or deformation to be possibly caused at the photoresist patterns in advance in consideration of optical-proximity effects. This process is called optical-proximity-effect corrections (the “OPC”).
An OPC operation is carried out by using exposure simulators, which provide photoresist patterns in response to input data of photomask-patterns on a simulation basis.
The exposure simulators, however, basically cannot correct a line width of photomask-patterns, for example. Photomask design engineers make new photomask-patterns with a line width correction, for example, which are, in turn, supplied to the exposure simulators to check photoresist patterns and such operations are necessarily repeated to obtain preferable photoresist patterns. Obviously, it takes a long time to correct photomask-patterns and is burdensome for the engineers to fix them satisfactorily.
In order to overcome such technical difficulties, a prior art photomask-pattern correction device has been proposed in Japanese Unexamined Patent Publication Tokkai Hei 11-218899, for example. The prior art photomask-pattern-correction device will be briefly explained below with reference to FIG. 20.
The photomask-pattern-correction device performs the OPC operation as follows:
Exposure parameters, photomask-pattern-correction data and photomask parameters are provided to the photomask-pattern-correction device (Step 2101). The exposure parameters are, for example, designed photomask-patterns to be transferred to a photoresist film, exposure wave-length and intensity at an exposure process, and focus positions.
A bias process (Step 2102) is then carried out based on the photomask-pattern parameters with respect to the designed photomask-patterns provided at Step 2101. The bias process enlarges and decreases the photomask-patterns in the case of positive and negative values with respect to their order, respectively, but leaves the photomask-patterns as they are in the case of nil.
Subsequently, exposure simulation (Step 2103) is executed in terms of the photomask-patterns which has been subjected to the bias process in Step 2102, and deviations between the designed photomask-patterns and photoresist patterns transferred from them are detected and assessed (Step 2104) as output result data. In this process, since a certain fixed bias is simply applied to the designed photomask-patterns, the transferred photoresist patterns deviate to some extent from the designed photomask-patterns.
OPC operation (Step 2105) is executed based on the deviations detected in Step 2104 and such corrected photomask-patterns are then subjected to exposure simulations (Step 2106). Deviations of the transferred photoresist patterns further modified in Step 2106 from the designed photomask-patterns are assessed and detected (Step 2107). If such assessed results from Step 2107 are better than then optimum OPC patterns which have been stored before, the OPC patterns obtained from Step 2105 are newly stored as optimum OPC patterns (Step 2108).
Executions from Step 2105 through Step 2108 are repeated until predetermined conditions are satisfied by them so that the optimum OPC patterns are determined at that time.
Finally, photomask-pattern verification (Step 2109) is performed for the OPC patterns stored at Step 2108 as to whether their line widths and distances comply with design rules. After the correction of errors, if any, exposure simulation (Step 2110) is executed. Final assessment (Step 2111) is made for outputs of Step 2110 to complete the OPC operation.
Such a prior art photomask-pattern-correction device is configured to automatically obtain optimum OPC patterns but corrected data verification is not described in detail. In the OPC operation of photomask-patterns, one of its targets is to make OPC patterns but its output patterns do not always result in ideal ones for all the patterns, i.e., the OPC patterns are not always consistent with photoresist patterns transferred to a photoresist film. Thus, it is a realistic approach to set up certain critical conditions so that OPC patterns cannot be used unless and until the OPC patterns satisfy the critical conditions.
In short, a prior art photomask-pattern correction device is merely provided with a function to make up OPC patterns and it is insufficient to verify functions as to whether pattern data are truly suitable for the transfer to a photoresist film.